Genome segments accumulate with different frequencies in Bombyx mori bidensovirus

Bombyx mori bidensovirus (BmBDV) of the family Bidnaviridae is the unique animal multipartite virus until now, which contains two-segmented linear single-stranded DNA genome VD1 (6.5 kb) and VD2 (6.0 kb). Both two segments and their complementary strands are encapsidated separately. Although the different frequencies of genome segments were reported in multipartite viruses of plants, the relative abundances of segments in the animal multipartite virus has not been studied yet. In this paper, the relative frequencies of VD1 and VD2 composing the genome of BmBDV were determined by qPCR in different samples. The results showed that the copy number of VD2 is 1.22-, 1.47-, and 1.73-fold as much as that of VD1 in purified virions, excrement of BmBDV-infected silkworm, and BmBDV-infected midguts, respectively. These results demonstrated that the various genome segments also accumulate with different frequency in BmBDV populations and the copy number of short-segment VD2 is more than that of long-segment VD1, indicating that the imbalance of segmental genome copy number is a common phenomenon in multipartite viruses of plants and animals.